We made cDNA copies of genes of three different rotavirus strains (the human Wa [serotypic 1], the bovine NCDV and the simian rhesus Rh2 [serotype 3]) were prepared by reverse transcribing their genomic RNAs or single stranded RNAs synthesized in vitro from rotavirus particles. The cDNAs were tailed and inserted into pBR322 and the resulting recombinants were used to transform E. coli. The genes from which most of these clones were derived were identified by means of a dot hybridization assay. Clones containing copies of each rotavirus gene (except for genes 10 and 11) have been identified. The size of the rotavirus cDNA inserts in the plasmids were determined for more than half of all the clones obtained (more than 2500). Rotavirus cDNAs that may represent full size copies of genes 5-9 have been identified and their restriction patterns analyzed. Some of them are currently being sequenced in order to deduce the amino acid sequences of the proteins they encode and to facilitate their insertion into expression vectors. Synthesis of the relevant protective antigens in bacteria or yeast directed by expression vectors containing cloned DNA copies of the appropriate rotavirus genes may provide abundant amounts of antigen for use in a vaccine. Expression of such antigens as fusion proteins on the surface of bacteria which could transiently colonize the small intestine may stimulate local immunity and provide protection. Alternatively, synthetic peptides representing the major antigenic determinants responsible for the induction of neutralizing antibody could be used for immunoprophylaxis.